Chemical compounds converted into pearl form are better known in the prior art by the name of "prills". In most cases they relate to inorganic products exhibiting no phenomenon of degradation either by heat or by moisture. The conversion into pearl form is simply carried out for convenience in use, because the pearls have a uniform distribution with regard to the diameter and the shape of the particles.
The manufacture of these pearls is described especially in patents EP 277,508, U.S. Pat. No. 4,525,198, and U.S. Pat No. 4,389,356. Processes for the preparation of these pearls consist, in a first stage, in melting the compound which is to be formulated and then passing it through a die or a perforated plate which is subjected at its base to flows which make it possible to form pearls or beads which fall in a tower countercurrentwise to a gas or to air, thus permitting the solidification of the beads, which do not adhere to the walls of the tower.
This process is widely employed for converting fertilizers (urea, nitrogen compound, phosphorus compound, and the like) because these products can be subjected to a violent heating to make them melt and to a fast cooling to ensure their solidification. In addition, the dimensional uniformity of the pearls (approximately a few millimeters) is not such a draconian condition in the fertilizer industry as in the pharmaceutical industry, where pearl dimensions of the order of a few tens of to a few hundred microns are desired.
When melted, some active substances exhibit so-called "supercooling" phenomena which considerably delay their solidification, even after cooling. The problem which the present invention has attempted to solve is that of converting into bead form those medicinal active substances which exhibit an indefinite crystallization point (or more commonly a supercooling phenomenon) and which therefore do not lend themselves well to the "prilling" technique because they tend to remain in an oily or pasty form well after returning to a temperature below their melting temperature. On passing compounds of this type through a prilling tower it was expected to have a pasty mass at the bottom of the tower or on the walls. The technique which consisted in blowing a jet of very cold air (-10.degree. C. to -20.degree. C.) over the jet of molten substance leaving the nozzle did not enable the problem to be solved. In fact, when supercooled products are brought into contact with an intense cold, the viscosity of the liquid increases considerably and slows down the subsequent crystallization. It was therefore necessary to avoid at all costs introducing a jet of excessively cold air at the exit of the nozzle.